In our South African Pat. No. 80/0224, we described a process wherein zinc sulphide containing material may be subjected to a two-stage leach process. In the first stage the ground material is leached, under pressure, with an aqueous solution having a high iron content and a low acid content. In the second stage the partly leached material together with precipitated iron, is leached with spent electroyte which has a high acid content. Both leaching stages are carried out at an elevated temperature which is below the melting point of sulphur (i.e. 119.degree. C.), and pressure, the partial pressure of oxygen being one of the important factors.
The process as described in South African Pat. No. 80/0224 has the disadvantage that an excess of zinc bearing sulphide mineral must be present to achieve acceptable leaching rates under elevated pressure. It is thus necessary to separate the unleached sulphide mineral from the final residue which contains a mixture of sulphides, gangue, elemental sulphur and any iron precipitate present, in order to recycle the unleached zinc sulphide mineral back to the first stage leach. It is also necessary to separate the elemental sulphur from the final multi-component residue mixture.
The present invention represents an improvement of the process described in South African Pat. No. 80/0224, and enables metal values to be recovered from zinc bearing sulphide minerals in such a way that the necessity to separate unleached sulphide mineral from a multi-component residue, in order to recycle unleached mineral may be eliminated.
Thus an object of this invention is to provide a process of the general type described wherein an excess of sulphide mineral is not necessary in order to achieve high dissolutions whilst also retaining the advantages of the process of our said earlier patent.